This invention relates to threaded fastener assemblies and, more particularly, to assemblies in which the fastener is captivated within the workpiece.
1. Discussion
Manufacturers are constantly striving to increase productivity by reducing assembly time during the manufacturing process. It is known in the art to streamline manufacturing by pre-attaching the fasteners to structural members which are later used in the assembly of a component.
FIGS. 1 and 2 illustrate a known captivated fastener assembly which is used to secure a valve cover to an engine cylinder head. The assembly 10 generally includes three pieces: fastener 12, sleeve 14 and grommet 16. It is necessary that the fastener 12 be able to slide axially within the bore of sleeve 14. On the other hand, it is also important that the assembly 10 remain assembled and not fall apart during shipment or when the cover is mounted to the engine cylinder head. Thus, there is a requirement that the fastener be "captivated" within the workpiece. A common commercially used design for accomplishing these goals employs an enlarged ring 18 which is formed on the fastener shank which cooperates with a reduced diameter section 20 formed in the sleeve 14. The outer diameter of the ring 18 and inner diameter defined by the sleeve section 20 are such that the fastener can be pushed through the sleeve so that the ring 18 falls below section 20 yet there still remains a sufficient interference between the ring 18 and section 20 to prevent the ring (and thus the fastener) from again passing upwardly past section 20 which otherwise would permit separation of the fastener from the sleeve 14.
This design requires special forming steps to generate the ring 18 on the fastener and the reduced diameter section 20 on the sleeve. Typically, the ring 18 is formed during the thread rolling process of the fastener and the section 20 is formed with a punch and die operation. It is very important that tight tolerances be kept during these forming processes. If the difference between the relative diameters is too great, then it is difficult and sometime not possible to insert the fastener 12 into the sleeve 14. On the other hand, if the ring outer diameter is too small relative to the section 20 inner diameter then the fastener can become separated from the sleeve.
Therefore, it would be desirable to provide a captivated fastener assembly that overcomes one or more of these problems.